Shortwave Irradiance (1950 to 2020): Dimming, Brightening, and Urban Effects in Central Arizona?

The objective of this study was to evaluate long-term change in shortwave irradiance in central Arizona (1950–2020) and to detect apparent dimming/brightening trends that may relate to many other global studies. Global Energy Budget Archives (GEBA) monthly data were accessed for the available years 1950–1994 for Phoenix, Arizona and other selected sites in the Southwest desert. Monthly data of the database called gridMET were accessed, a 4-km gridded climate data based on NLDAS-2 and available for the years 1979–2020. Three Agricultural Meteorological Network (AZMET) automated weather stations in central Arizona have observed hourly shortwave irradiance over the period 1987–present. Two of the rural AZMET sites are located north and south of the Phoenix Metropolitan Area, and another site is in the center of the city of Phoenix. Using a combination of GEBA, gridMET, and AZMET data, annual time series demonstrate dimming up to late 1970s, early 1980s of −30 W/m2 (−13%), with brightening changes in the gridMET data post-1980 of +9 W/m2 (+4.6%). An urban site of the AZMET network showed significant reductions post-1987 up to 2020 of −9 W/m2 (3.8%) with no significant change at the two rural sites.